1. Field of the Invention
The present invention relates to measuring and control of fluid flow, particularly at low fluid flow rates. More particularly, the present invention permits such measurement and control without introducing measuring sensors or devices into the path of the fluid flow.
2. Description of the Related Art
Measurement of the flow or flow rate of a fluid in a conduit, particularly, at very low fluid flow rates, has been a problem if attempted using conventional flow sensors. At very low flow many fluid sensors do not operate properly. For example, velocity flow meters such as turbine wheel flow sensors cease to operate due to there being insufficient energy in the fluid to rotate the wheel. Differential pressure flow sensors can at times operate at low flows, but the smaller flow orifices required for low flows have been prone to obstruction if there were suspended particles in the fluid. Also, pressure drops across the orifices could be significant.
Most thermal flow sensors have the temperature sensing mechanism as a resistance bridge circuit or as a part of the entire temperature variant area whereby a change in flow has the equal and opposite effect on the two halves of the sensor. This can have a limiting effect on the range of the sensor and ambient temperature changes can affect the accuracy.
Specialized flow sensing techniques have been attempted in certain cases. For example, U.S. Pat. No. 5,035,138 used a resistive material formed of a special alloy as a tube or conduit through which a gas flowed. The special alloy was selected because of a high electrical resistivity and a high temperature coefficient of resistance. A voltage differential was applied to the resistive alloy conduit at defined positions. The resistive material of the conduit was used to heat the fluid flowing in it according to the voltage differential applied. The resistive alloy material tube acted both to heat the fluid and as an indicator of flow conditions. Spaced portions of the tube wall served as temperature sensitive resistors which developed a voltage differential as flow rates through the tube varied. The flow sensor of this patent required that the fluid tube or conduit be formed of a special resistive alloy having a high change in resistance value as a function of temperature change.
Briefly, the present invention provides a new and improved system and method for measuring the flow of fluid in a conduit which is transporting the fluid. A set of at least two heat detectors is mounted on the conduit at spaced positions from each other on the conduit. The heat detectors measure the temperature of the fluid and conduit at the spaced positions. A set of at least two power applicators is mounted with the conduit at different locations along the conduit. The power applicators apply electrical power to the conduit to heat the conduit and the fluid in the conduit at an established temperature differential between the different locations. A control mechanism measures the level of power furnished to the power applicators to maintain the established temperature differential, and thus provide an indication of the flow rate based on the energy measured by the control mechanism.
The present invention is also adapted to control the flow of fluid at a measured rate of flow. A flow regulating valve responsive to the measure of the fluid flow is provided to control the flow of fluid to a desired flow rate based on the measured fluid flow rate.
The present invention does not require that any sensing devices be placed in the path of fluid flow in the conduit, and thus also affords a straight-through flow design for the conduit. The present invention requires only a small temperature differential for flow rate measurement and does not degrade temperature sensitive fluids.
The present invention also makes flow rate measurements while causing a relatively low pressure drop. The conduit in which flow is measured according to the present invention is formed of an electrically conductive material such as stainless steel, and the interior of the conduit may be coated with a suitable corrosion-resistant synthetic resin film.